This invention relates to wireless communication transmitters, and more particularly to methods and radio transmitters that use pseudo-polar modulation to modulate phase and amplitude of an information signal.
It has long been known that any type of radio carrier modulation can be described in terms of a complex modulation, and the complex modulation can in turn be described either in terms of Cartesian (Real and Imaginary) waveform components or in terms of Polar (Amplitude and Phase) components. A single sideband signal (which varies in both phase and amplitude) can be formed from a constant-amplitude signal that is modulated in phase angle, with the phase part of the SSB signal and then modulating the constant-amplitude signal in amplitude with the amplitude part of the SSB signal.
There are many other forms of modulation apart from SSB that consist of varying phase and amplitude. One way to contain the transmission of a signal to an allocated channel, with minimum spectral spillover to adjacent channels, involves amplitude variation. This is because a signal modulated in phase alone is not perfectly bandlimited. A linear power amplifier is generally used to more faithfully reproduce the amplitude variations for containing the transmission spectrum. The linear power amplifier must pass the amplitude peaks without distortion, and therefore operates with a mean that is lower than its maximum output, and which may be lower than its maximum efficiency. Such linear amplifiers still however produce some distortion even when operated below their clipping point, because their amplitude-in to amplitude-out transfer characteristics are not completely linear.
Polar modulation may be used to reduce such distortion. Using polar modulation, an amplifier operates with a constant amplitude drive signal, and therefore does not need to have a linear amplitude-in to amplitude-out characteristic. The desired amplitude modulation is then applied after amplification, for example by modulating the power supply to the amplifier. However, this can give rise to another form of distortion, called AM-to-PM conversion, in which the amplitude modulation causes an inadvertent phase modulation. Feedback and feedforward techniques may be used to compensate for the known AM-PM tendency by adjusting phase modulation that is dependent on the amplitude modulation.
Polar modulation is supposed to produce a transmission that is more tightly spectrally contained than either a purely phase-modulated signal or a purely amplitude-modulated signal would be alone. Ideally, the out-of-band components of the amplitude modulation part cancel the out-of-band components of the phase modulation part. However, complete cancellation is prejudiced by errors in either part, such as due to different bandwidth restrictions, delay in the amplitude modulator compared to the phase modulator, and/or limited sampling rates.